1. Field of the Invention
The present invention relates to separators between adjacent electrochemical cells. More particularly, the invention relates to lightweight bipolar plates and methods for their construction.
2. Background of the Invention
Electrochemical cells utilizing a proton exchange membrane (PEM) can be configured in cell stacks having bipolar separator plates between adjacent cells. These bipolar separator plates are typically made from a variety of metals, such as titanium and stainless steel, and non-metallic conductors, such as graphitic carbon. Bipolar separator plates can be fabricated by machining fluid flow fields into a solid sheet of the material. The flow fields are made up of a series of channels or grooves that allow passage of gases and liquids.
FIG. 1 is a face view of a prior art bipolar separator plate 10 made from a solid sheet of a conducting material. The central portion of the plate has a flow field 12 machined into its surface. The flow field may direct fluid flow in many patterns, but is illustrated here as parallel serpentine channels. Around the perimeter of the flow field 12, the plate provides a plurality of bolt holes 14 for assembling and securing a cell stack, various manifolds 16 for communicating fluids in and out of the stack, and a flat surface 18 that allows the plate to be sealed with adjacent components of the cell stack.
In addition to providing a fluid flow field, a bipolar separator plate for use in electrochemical cells must collect electrons liberated at one electrode, conduct the electrons through the plate, and deliver electrons to the face of another electrode on the opposing side of the plate. The prior art bipolar separator plate collects and delivers electrons from electrodes of opposing cells through the ridges 20 remaining between the channels 22 in the flow field 12.
FIG. 2 is a schematic view of a proton exchange membrane (PEM) electrochemical cell 30 configured as a hydrogen-air fuel cell. This stack comprises two identical fuel cells 32 having a cathode 34, a PEM 36 and an anode 38. Flow fields 40 (shown schematically for clarity) are provided on either side of the bipolar separator plate 42, as well as on the internal faces of the endplates 44. Electrons liberated at the anode 38 provide current flow to the cathode 34 of a cell on the other side of the plate 42 and, ultimately, through an external circuit 46. Electrons are then combined with protons and oxygen at the cathode 34 to form water. The electrical potential of the fuel cell 30 is increased by adding more cells 32 to the stack.
Weight is a characteristic of electrochemical cells generally, and fuel cells in particular, that limits their use. Therefore, significant efforts have been directed at providing lightweight components for electrochemical cells. Even so, there remains a need for a lightweight bipolar separator plate. It would be desirable if the lightweight bipolar separator plate could also be made thinner and support higher current densities. It would be further desirable if the structure of the bipolar separator plate allowed the introduction of other specific properties, such as water permeability and reactant gas impermeability.
The present invention provides a separator for electrochemical cells comprising a gas barrier having an electrically. conducting pathway extending therethrough and a porous, electrically conducting member in electrical contact with each side of the electrically conducting pathway, the member selected from the group consisting of expanded metal mesh, metal foam, conducting polymer foam, porous conductive carbon material and combinations thereof. The electrically conducting pathway through the gas barrier may be formed from a second porous, electrically conducting member selected from the group consisting of expanded metal mesh, metal foam, conducting polymer foam, porous conductive carbon material and combinations thereof. The gas barrier may be comprised of a metal sheet or a polymer filled porous, electrically conducting member.
In another aspect of the invention, a separator for electrochemical cells is provided comprising a porous, electrically conducting sheet selected from the group consisting of expanded metal mesh, metal foam, conducting polymer foam, porous conductive carbon material and combinations thereof and a gas impermeable material, such as a polymer or metal, disposed within a portion of the sheet to form a gas barrier. The gas impermeable polymers are selected from the group consisting of water permeable polymers, thermoplastic polymers, reactively cured polymers, and combinations thereof.
In yet another aspect of the invention, a separator for electrochemical cells is provided comprising two porous, electrically conducting sheets selected from the group consisting of expanded metal mesh, metal foam, conducting polymer foam, porous conductive carbon material and combinations thereof and an electrically conducting gas barrier disposed in electrical contact between the sheets.